Videos
The seminar starts every Thursday at 17:00 CET or CEST (Central European Time or Central European Summer Time, depending on the time of year) and is livestreamed on our youtube channel.
Once the live stream has finished, we will make all video recordings and .pdf versions of the presentation slides available here.
Thu 20210114 17:00 CET
Quantum Science Seminar #31: Quantum Repeaters
Institute of Photonic Sciences (ICFO)
Barcelona — Spain
Barcelona — Spain
Quantum Nodes for Quantum Repeaters
The distribution of entanglement between the nodes of
a quantum network will allow new advances e.g. in long
distance quantum communication, distributed quantum
computing and quantum sensing. On the ground, quantum
information can be distributed across the nodes using
single photons at telecommunication wavelengths
traveling in optical fibers. To bridge distances much
longer than the fiber attenuation length, quantum
repeaters can be used. The nodes of a quantum network
are matter systems that should efficiently interact
with quantum light, allow entanglement with photons
(ideally at telecommunication wavelengths) and serve
as a quantum memory allowing longlived and faithful
storage of (entangled) quantum bits. In addition, for
efficient distribution of entanglement, the nodes
should allow multiplexed operation and ideally enable
quantum processing capabilities between stored qubits.
While several candidates are actively investigated,
none so far regroups all the desired functionalities,
highlighting the need for hybrid quantum networks
connecting disparate quantum nodes with complementary
capabilities. In this talk, after introducing the
context I will describe our recent progress towards
the realization of quantum repeater nodes with
multiplexed ensemblebased quantum memories, using
cryogenically cooled rareearth ion doped solids and
lasercooled cold atomic gases. I will also describe
our efforts to distribute quantum information between
disparate quantum nodes and to scale up quantum
network links including lightmatter and mattermatter
entanglement experiments. In particular, I will report
a recent experiment demonstrating entanglement between
remote multiplexed solidstate quantum memories,
heralded by a telecom photon. Finally, I will explain
our current work to build quantum processing nodes
using e.g. single rareearth ions in microcavities or
ensembles of cold Rydberg atoms.
Thu 20201217 17:00 CET
Quantum Science Seminar #30: Precision Measurement
Universität Mainz
Mainz — Germany
Mainz — Germany
Gamma Factory: a new photonic tool for science
The Gamma Factory (GF) is an ambitious proposal
developed as part of the CERN Physics Beyond Colliders
program to operate the LHC as a novel kind of a light
source that can produce up to ~10^{17} photons
per second with energies of up to ~400 MeV. The key
idea is to scatter laser photons off of partially
stripped ultrarelativistic ions. The GF would enable a
number of hitherto impossible experiments and can be
thought of not only as a unique photon source, but also
as a giant ion trap for precision physics. The first
step in practical realization of the GF is a
fullfeatured proofofprinciple experiment at the SPS
that is currently being considered by CERN.
Thu 20201210 17:00 CET
Quantum Science Seminar #29: Rydberg Atoms
Durham University
Durham — United Kingdom
Durham — United Kingdom
Hybrid quantum interfaces using Rydberg collective encoding
Rydberg atoms has emerged as a versatile platform for
different applications in quantum technology from
computing and simulation to sensing and imaging [1].
In this talk, I will focus on one of the many Rydberg
projects in Durham. In our Rydberg quantum optics
experiment, we store optical photons in a cold atomic
ensemble in the form of Rydberg polaritons. In recent
work, we have looked at the potential of Rydberg
polaritons in the context of quantum information. We
show that Rydberg polaritons have a number of
attractive features. In particular, the combined
atomic and photonic character of the polariton [2]
allows coherent mapping between static and flying
qubits, the large dipole moments between Rydberg
states enables fast single qubit rotations, and as the
quantum information is shared amongst many atoms,
there is an inbuilt robustness to atom loss [3].
Finally, the robustness of collectivelyencoded
Rydberg qubits to environmental noise is considered.
Thu 20201203 17:00 CET
Quantum Science Seminar #28: Circuit QED
Yale University
New Haven — Connecticut — U.S.A.
New Haven — Connecticut — U.S.A.
Controlling Bosonic Modes in Circuit QED and the Application to Vibronic Molecular Simulations
Circuit quantum electrodynamics, in which microwave
cavity modes are coupled to “artificial atoms”
realized with Josephson junction qubits, has allowed
for a variety of investigations in quantum optics and
quantum information. In recent years, our team at Yale
has focused on a hardware efficient approach, where
highQ microwave cavities serve as quantum memories.
When dispersively coupled to transmon qubits, quite
complex nonclassical states can be created in these
cavities, and operations between cavities can be
enacted through parametric driving or other means. For
instance, we have recently shown highquality
cavitycavity swaps via a beamsplitter or conversion
operation, single and twomode squeezing, and
engineered cross and self Kerr interactions. Finally,
one can perform strong projective measurements of the
photon number, the photon parity, or indeed any other
binaryvalued operator within the multidimensional
Hilbert space. This system therefore has all of
capabilities of linear optical systems, but with the
addition of deterministic state preparation,
measurement, and nonlinear interactions. One way to
employ these capabilities is to directly simulate
problems which are “naturally” bosonic in nature, to
calculate vibronic spectra, FranckCondon factors, or
nonlinear molecular dynamics. I will present
preliminary work towards using this platform as a
novel but programmable simulator for small molecules.
Thu 20201126 17:00 CET
Quantum Science Seminar #27: Mesoscopic Systems
University of Innsbruck and
IQOQI, Austrian Academy of Sciences
Innsbruck — Austria
IQOQI, Austrian Academy of Sciences
Innsbruck — Austria
Levitated Nanoparticles in the Quantum Regime
During the last ten years, a fastgrowing scientific
activity has been devoted to understanding,
controlling and using the dynamical degrees of freedom
of levitated solidstate objects in vacuum. Levitated
nanoparticles can move, rotate and fall. They also
have solidstate internal degrees of freedom. At the
interface between the fields of cavity optomechanics
and atomic and molecular optics, the rich physics of
levitated nanoparticles provide unique opportunities
for applied and fundamental research.
In this talk, we will first motivate and review the
research activities in this field. We will then focus
on two recent results in our theory group. First, we
will discuss how the centerofmass motion of a
micromagnet can be coupled to its internal acoustic
phononic modes. This offers a new cooling method and a
tool to probe and control the internal physics of an
isolated micromagnet. Then, we will discuss how to
optimally control the harmonic potential of a
levitated nanoparticle to quantumdelocalize its
centerofmass motional state. This has applications
for testing quantum mechanics at large scales,
enhancing sensitivities of force sensors, and boosting
the entangling rate of two weakly interacting
nanoparticles.
References
Thu 20201119 17:00 CET
Quantum Science Seminar #26: Quantum Control
Freie Universität Berlin
Berlin — Germany
Berlin — Germany
Training Schrödinger's Cat: Quantum Control in Molecular Physics and Quantum Information Science
Control refers to the ability to steer a dynamical
system using external fields; quantum control does so
by exploiting quantum coherence, i.e., the wave nature
of matter. One way to think of it is in terms of
constructive and destructive interference between
different quantum pathways, all connecting the same
initial and final states. I will illustrate the
concept of pathway interference using the
photoionization of chiral molecules, i.e., molecules
with a lefthanded or righthanded nuclear scaffold,
as example. The ionizing field may be tailored to
minimize or maximize the signature of molecular
handedness in the photoelectron spectrum, using
interference between pathways probing different
intermediate states.
The essential elements of quantum physics, quantum coherence and entanglement, are not only the agents of quantum control, they are also at the core of emerging quantum technologies such as quantumenhanced sensing or quantum information processing. I will discuss how quantum control allows to identify fundamental performance bounds and derive protocols to reach these performance bounds in realistic models for basic building blocks of quantumenhanced sensing and quantum information processing.
The essential elements of quantum physics, quantum coherence and entanglement, are not only the agents of quantum control, they are also at the core of emerging quantum technologies such as quantumenhanced sensing or quantum information processing. I will discuss how quantum control allows to identify fundamental performance bounds and derive protocols to reach these performance bounds in realistic models for basic building blocks of quantumenhanced sensing and quantum information processing.
Thu 20201112 17:00 CET
Quantum Science Seminar #25: Dark Matter Searches
University of Delaware
Newark — Delaware — U.S.A.
Newark — Delaware — U.S.A.
Dark Matter Searches with Atomic and Nuclear Clocks
The extraordinary advances in quantum control of matter
and light have been transformative for precision
measurements enabling probes of the most basic laws of
Nature to gain fundamental understanding of the physical
Universe. Exceptional versatility, inventiveness, and
rapid development of precision experiments supported by
continuous technological advances and improved theory
give a high chance for paradigmshifting discovery. The
development of atomic clocks with systematic
uncertainties in the 10^{18} range enables searches for
the variation of fundamental constants, dark matter, and
violations of Lorentz invariance. I will give an
overview of dark matter searches with clocks including
prospects for significantly improved sensitivity with
highly charged ions and a nuclear clock. At the end, I
will also introduce our new online portal for
highprecision atomic data and computations.
References

The search for variation of fundamental constants with clocksAnnalen der Physik53118003642019

Search for new physics with atoms and moleculesReviews of Modern Physics900250082018

Accurate Prediction of Clock Transitions in a Highly Charged Ion with Complex Electronic StructurePhysical Review Letters1241630012020
Thu 20201105 17:00 CET
Quantum Science Seminar #24: Young Researchers
SISSA — International School for Advanced Studies
Trieste — Italy
Trieste — Italy
Quantum simulation of lattice gauge theories with Rydberg atoms
Gauge theories are the cornerstone of our
understanding of fundamental interactions among
particles. Their properties are often probed in
dynamical experiments, such as those performed at ion
colliders and highintensity laser facilities.
Describing the evolution of these strongly coupled
systems is a formidable challenge for classical
computers, and represents one of the key open quests
for quantum simulation approaches to particle physics
phenomena. In our work, we show how recent experiments
done on Rydberg atom chains naturally realize the
realtime dynamics of a lattice gauge theory at system
sizes at the boundary of classical computational
methods.
Technion
Haifa — Israel
Haifa — Israel
Observation of a smooth polaron–molecule transition in a degenerate Fermi gas
Understanding the behavior of an impurity
stronglyinteracting with a Fermi sea is a
longstanding challenge in manybody physics. For a
single impurity, a firstorder transition is predicted
between polaronic quasiparticle and dressed molecular
ground states. However, at realistic conditions, the
fate of this transition is still unknown. We study
imbalanced ultracold Fermi gas with a novel
highsensitivity Raman spectroscopy technique to
isolate the quasiparticle contribution and extract
physical parameters. We observe a reduction of the
quasiparticle weight with increasing interaction and
support this finding with a theoretical model of
thermally occupied quasiparticles, revealing a smooth
transition with a coexistence region.
Universität Innsbruck
Innsbruck — Austria
Innsbruck — Austria
Crossverification of quantum devices
With quantum computers on the brink of outperforming
their classical counterpart, it becomes increasingly
important to find ways of verifying that these devices
indeed perform as advertised. We approach this task by
challenging independent quantum processors with
seemingly random sampling problems of different sizes,
which are linked through the principles of
measurementbased quantum computing. This enables
efficient crosschecking of multiple quantum
processors, as well as internal consistency checks of
individual quantum processors. The protocol is
hardware agnostic and requires no classical simulation
as we demonstrate using five stateoftheart quantum
processors.
Thu 20201029 17:00 CET
Quantum Science Seminar #23: NV Centers
Institut für Theoretische Physik, Universität Ulm and IQST
Ulm — Germany
Ulm — Germany
Colour centers in diamond: From quantum simulation to nanoscale NMR
Perfect diamond is transparent for visible light but
there are famous diamonds, such as the famous Oppenheim
Blue or the Pink Panther worth ten's of millions of
dollar, which have intense colour. An important source
of colour in diamond are lattice defects which emit and
absorb light at optical frequencies and may indeed
possess a nonvanishing ground state electronic spin.
Unlike atomic physics systems, which are operated under
extreme conditions to ensure isolation from their
environment, colour centers in diamond are by their very
nature in direct contact with the environment that is
constituted by uncontrolled phonon, spin and charge
degrees of freedom that form part of the host material.
As a result they suffer from environmental noise. In
order to address this challenge a combination of
material science to improve the hardware properties and
of quantum control methods to further decouple the solid
state qubits from their environment while maintaining
desired interactions that are needed to achieve the
ultimate goal of practically usable coherent quantum
dynamics. In this lecture I will explore the physics of
one of these defects, the nitrogen vacancy center, and
show how we can manipulate its electronic spin to
develop quantum simulators, nanoscale quantum sensors
and sources of nuclear hyperpolarisation. Applications
of such devices range from sensing in biology to medical
imaging.
References

A largescale quantum simulator on a diamond surface at room temperatureNature Physics91682013

Submillihertz magnetic spectroscopy performed with a nanoscale quantum sensorScience3568322017

Robust optical polarization of nuclear spin baths using Hamiltonian engineering of nitrogenvacancy center quantum dynamicsScience Advances4eaat89782018

Blueprint for Nanoscale NMRScientific Reports969382019
Thu 20201022 17:00 CEST
Quantum Science Seminar #22: Quantum Metrology
Niels Bohr Institute, University of Copenhagen
Copenhagen — Denmark
Copenhagen — Denmark
Quantum mechanics in the negative mass reference frame
According to textbook quantum mechanics, a measurement
of the position of an object imposes a random quantum
back action kick to its momentum. This randomness
translates with time into position uncertainty, thus
leading to the well known uncertainty on the
measurement of motion. As a consequence, and in
accordance with the Heisenberg uncertainty principle,
this leads to the socalled standard quantum limit on
the precision of sensing of position, forces and
fields. In this talk I will present the ideas and
experimental results for measurement of motion of a
mechanical oscillator with, in principle, unlimited
precision. This is achieved by measuring the motion in
a special reference frame with an effective negative
mass. Entanglement of the system of interest and the
reference frame leads to cancellation of the quantum
back action of the measurement which is the origin of
the standard quantum limit. Applications to force
sensing, and gravitational wave detection will be
discussed.
Thu 20201015 17:00 CEST
Quantum Science Seminar #21: Quantum Gases
5. Physikalisches Institut and IQST
Universität Stuttgart
Stuttgart — Germany
Universität Stuttgart
Stuttgart — Germany
A dipolar supersolid and a novel microscope to probe quantum gases
Dipolar interactions are fundamentally different from
the usual van der Waals forces in real gases. Besides
the anisotropy the dipolar interaction is nonlocal and
as such allows for self organized structure
formation [1]. Similar to the Rosensweig
instability in classical magnetic ferrofluids
selforganized structure formation was expected.
However on the meanfield level such a transition is
instable due to the diluteness of the gaseous sample.
In contrast to these predictions in 2015 we could
observe the formation of a stable droplet crystal and
found that this unexpected stability is due to beyond
meanfield quantum corrections of the LeeHuangYang
type. When arranged in a 1D array also phase coherence
between the droplets was observed, which was first
evidence for a supersolid state of matter. Upon
crossing the transition to the dipolar supersolid a
Goldstone mode appears, which we have observed. The
existence of this mode proofs the superfluid stiffness
or the socalled phase rigidity of this new state of
matter. Recently we have also measured the static
structure factor across the transition which allows to
show that the characteristic fluctuations correspond
to elementary excitations such as the roton modes, and
that the supersolid state supports both superfluid as
well as crystal phonons. A recent review on the
discovery of quantum dropets and dipolar supersolids
can be found in Ref. [2]. At the end of the talk
I will quickly introduce a new approach to quantum gas
microscopy making use of a pulsed ion microscope with
resolution below 200 nm, very fast time resolution,
large field of view and depth of field and the
possibility to image in 3D [3].
Thu 20201008 17:00 CEST
Quantum Science Seminar #20: Quantum Simulation
Center for Quantum Physics, University of Innsbruck, and
IQOQI, Austrian Academy of Sciences
Innsbruck — Austria
IQOQI, Austrian Academy of Sciences
Innsbruck — Austria
Programmable Quantum Simulators with Atoms and Ions
Quantum optical systems with cold atoms and ions
provides one of the best ways to build controllable
quantum manybody systems as quantum computers and
quantum simulators. Here we report on recent
developments in building, and in particular
programming quantum simulators based on trapped ions
as intermediate scale quantum devices. Our discussion
will focus on hybrid classicalquantum scenarios: here
the quantum part is the generation of highly entangled
states on the quantum device in quench dynamics, which
is combined with a classical post processing of
measurement data, possibly run in a feedback loop with
the quantum device. Examples highlighting these
developments include the implementation of
selfverifying variational quantum simulations,
illustrated here by computing the ground state and
quantum phase transition of a Schwinger Model as 1D
QED. In addition, we develop and demonstrate a
‘randomized measurement toolbox’, allowing to access
in experiments quantities like Renyi entanglement
entropies and — as ongoing research — measure the
entanglement spectrum, i.e. ‘seeing the Schmidt
decomposition live’ in quench dynamics from an initial
product state towards thermodynamic equilibrium.
Thu 20200924 17:00 CEST
Quantum Science Seminar #19: Atom Arrays
Harvard University
Cambridge — Massachusetts — U.S.A.
Cambridge — Massachusetts — U.S.A.
Programmable quantum systems based on Rydberg atom arrays
We will discuss the recent advances involving
programmable, coherent manipulation of quantum
manybody systems using atom arrays excited into
Rydberg states. Specifically, we will describe our
recent technical upgrades that now allow the control
over 200 atoms in twodimensional arrays. Recent
progress involving the realization of exotic states of
matter, exploration of their nonequilibrium dynamics
as well as realization and testing quantum
optimization algorithms using such systems will be
discussed.
References

Probing manybody dynamics on a 51atom quantum simulatorNature5515792017

Quantum Kibble–Zurek mechanism and critical dynamics on a programmable Rydberg simulatorNature5682072019

Generation and manipulation of Schrödinger cat states in Rydberg atom arraysScience3655702019

Parallel implementation of highfidelity multiqubit gates with neutral atomsPhysical Review Letters1231705032019
Thu 20200917 17:00 CEST
Quantum Science Seminar #18: Precision Measurement
PhysikalischTechnische Bundesanstalt Braunschweig and
Leibniz Universität Hannover — Germany
Leibniz Universität Hannover — Germany
Quantum Logic Spectroscopy of Trapped Ions
Precision spectroscopy is a driving force for the
development of our physical understanding. However,
only few atomic and molecular systems of interest have
been accessible for precision spectroscopy in the
past, since they miss a suitable transition for laser
cooling and internal state detection. This restriction
can be overcome in trapped ions through quantum logic
spectroscopy [1]. Coherent laser manipulation
originally developed in the context of quantum
information processing with trapped ions allows us to
combine the special spectroscopic properties of one
ion species (spectroscopy ion) with the excellent
control over another species (logic or cooling ion).
The logic ion provides sympathetic cooling and is used
to control and read out the internal state of the
spectroscopy ion. In my talk I will provide an
overview of different implementations and applications
of quantum logic spectroscopy to investigate
previously inaccessible species such as molecular
ions [2] and highly charged ions [3].
Spectroscopy of these species may reveal physics
beyond the standard model, such as new force carriers
or scalar fields that are dark matter candidates and
could induce a variation of fundamental constants, or
appear as nonlinearities in isotope shift
spectroscopy.
Thu 20200910 17:00 CEST
Quantum Science Seminar #17: Laser Science
Weizmann Institute of Science
Rehovot — Israel
Rehovot — Israel
Solving computational
problems with coupled lasers
Computational problems may be solved by realizing
physics systems that can simulate them. Here we
present a new system of up to >1000 coupled lasers
that is used to solve difficult computational tasks.
The wellcontrolled dissipative coupling anneals the
lasers into a stable phaselocked state with minimal
loss, that can be mapped on different computational
minimization problems. We demonstrate this ability for
simulating XY spin systems and finding their ground
state, for phase retrieval, for imaging through
scattering medium and more.
Thu 20200903 17:00 CEST
Quantum Science Seminar #16: Quantum Simulation
MPQ, LMU, and MCQST
München — Germany
München — Germany
Quantum Simulations using Ultracold Quantum Matter
More than 30 years ago, Richard Feynman outlined his
vision of a quantum simulator for carrying out complex
calculations on physical problems. Today, his dream is
a reality in laboratories around the world. This has
become possible by using complex experimental setups
of thousands of optical elements, which allow atoms to
be cooled to Nanokelvin temperatures, where they
almost come to rest. Recent experiments with quantum
gas microscopes allow for an unprecedented view and
control of such artificial quantum matter in new
parameter regimes and with new probes. In our quantum
gas microscope experiments, we can detect both charge
and spin degrees of freedom simultaneously, thereby
gaining maximum information on the intricate interplay
between the two in the paradigmatic Hubbard model. In
my talk, I will show how we can reveal hidden magnetic
order, directly image individual magnetic polarons or
probe the fractionalisation of spin and charge in
dynamical experiments. For the first time we thereby
have access to directly probe nonlocal ‘hidden’
correlation properties of quantum matter and to
explore its real space resolved dynamical features
also far from equilibrium.
Thu 20200730 17:00 CEST
Quantum Science Seminar #15: Quantum Dynamics
JILA, NIST and University of Colorado
Boulder — Colorado — U.S.A.
Boulder — Colorado — U.S.A.
Observation of Dynamical Phase Transitions in Cold Atomic Gases
Nonequilibrium quantum manybody systems can display
fascinating phenomena relevant for various fields in
science ranging from physics, to chemistry, and
ultimately, for the broadest possible scope, life
itself. The challenge with these systems, however, is
that the powerful formalism of statistical physics,
which have allowed a classification of quantum phases
of matter at equilibrium does not apply. Therefore,
using controllable cold atomic systems to shed light
on the organizing principles and universal behaviors
of dynamical quantum matter is highly appealing. One
emerging paradigm is the dynamical phase transition
(DPT) characterized by the existence of a
longtimeaverage order parameter that distinguishes
two nonequilibrium phases. I will report the
observation of a DPT in two different but
complementary systems: a trapped quantum degenerate
Fermi gas and long lived arrays of atoms in an optical
cavity. I will show how these systems can be used to
simulate iconic models of quantum magnetism with
tunable parameters and to probe the dependence of
their associated dynamical phases on a broad parameter
space. Besides advancing quantum simulation our
studies pave the ground for the generation of
metrologically useful entangled states which can
enable real metrological gains via quantum
enhancement.
References
Thu 20200723 17:00 CEST
Quantum Science Seminar #14: Atom Arrays
Laboratoire Charles Fabry, Institut d’Optique, CNRS
Palaiseau — France
Palaiseau — France
Manybody physics with arrays of individual atoms
This talk will present our effort to control and use
the dipoledipole interactions between cold atoms in
order to implement spin Hamiltonians useful for
quantum simulation of condensed matter situations [1].
We trap individual atoms in arrays of optical tweezers
separated by few micrometers. We create almost
arbitrary geometries of the arrays with unit filling
in two and three dimensions up to about 70 atoms. To
make the atoms interact, we either excite them to
Rydberg states or induce optical dipoles with a
nearresonance laser.
We have demonstrated the coherent energy exchange in chains of Rydberg atoms resulting from their resonant dipoledipole interaction. This interaction realizes the XY spin model and leads to the hopping a spin excitation from a site to another. We use this interaction to study elementary excitations in a dimerized spin chain featuring topological properties (SuSchriefferHeeger model). We have observed the edge states in the topological condition. We probed the regime beyond the linear response by adding several excitations, which act as hardcore bosons [2].
With optical dipoles, we explore light scattering in one dimensional chains of atoms. This system realizes a dissipative spin model, which could find applications in quantum optics to generate optical nonlinearities and nonclassical states of light [3].
We have demonstrated the coherent energy exchange in chains of Rydberg atoms resulting from their resonant dipoledipole interaction. This interaction realizes the XY spin model and leads to the hopping a spin excitation from a site to another. We use this interaction to study elementary excitations in a dimerized spin chain featuring topological properties (SuSchriefferHeeger model). We have observed the edge states in the topological condition. We probed the regime beyond the linear response by adding several excitations, which act as hardcore bosons [2].
With optical dipoles, we explore light scattering in one dimensional chains of atoms. This system realizes a dissipative spin model, which could find applications in quantum optics to generate optical nonlinearities and nonclassical states of light [3].
References

Observation of a symmetry protected topological phase of interacting bosons with Rydberg atomsScience3657752019

ManyBody Physics with Individually controlled Rydberg AtomsNature Physics161322020

Collective shift in resonant light scattering by a onedimensional atomic chainarXiv2004.05395 (Phys. Rev. Lett., in press)2020
Thu 20200716 17:00 CEST
Quantum Science Seminar #13: Quantum Computing
University of Virginia
Charlottesville — Virginia — U.S.A.
Charlottesville — Virginia — U.S.A.
Quantum computing over the rainbow: the quantum optical frequency comb as a platform for measurementbased universal quantum computing
An ultrafast laser emits vastly multimode light over a
broad spectral band, a.k.a. the optical frequency comb
(OFC), but the emission happens but one photon at a
time, if in a stimulated manner, and no entanglement
is created in the light. Changing the gain medium from
linear (onephoton) to nonlinear (twophoton) yields
an optical parametric oscillator which features
massively multipartite entanglement of the OFC modes,
as demonstrated experimentally by our group and
others. This entanglement can then be exquisitely
tailored to cluster states with specific graphs, in
particular the twodimensional ones that are universal
for measurementbased, oneway quantum computing. It
is worth noting that this requires only sparse
experimental resources that are highly compatible with
integrated optics, thereby paving the way to the
realization of practical quantum computers.
References

Continuousvariable quantum computing in the quantum optical frequency combJournal of Physics B: At. Mol. Opt. Phys.530120012020

Experimental realization of multipartite entanglement of 60 modes of a quantum optical frequency combPhysical Review Letters1121205052014

Entanglement gets scaled up in an optical frequency combPhysics Today64212011
Thu 20200709 17:00 CEST
Quantum Science Seminar #12: Quantum Reform of SI
JQI, NIST and University of Maryland
College Park — Maryland — U.S.A.
College Park — Maryland — U.S.A.
A New Measure: the quantum reform of the International System of Units
The metric system began with the French revolution,
with the lofty ideal that measurements would be tied
to the size of the earth, universally available to
all. Soon, practical considerations required units of
length and mass based on unique physical artifacts, a
nearantithesis to universal availability. Now we are
experiencing the greatest revolution in measurement
since the French revolution, a revolution rooted in
the atomic and quantum view of nature, again offering
universal availability. The definitions of the
kilogram, ampere, kelvin, and mole were all changed on
20 May 2019, and are now based on chosen and fixed
values for Planck’s constant, the quantum of electric
charge, Boltzmann’s constant, and Avogadro’s number. I
will explain how this is possible, why it was
necessary, and speculate about future changes in the
SI. In this context I will also discuss the role of
precision measurement in the history and future of
quantum physics.
Thu 20200702 17:00 CEST
Quantum Science Seminar #11: Nanophotonics
Humboldt University
Berlin — Germany
Berlin — Germany
Revisiting LightMatter Interaction in Quantum Nanophotonics
The interaction of a singlemode light field with a
single atom or an ensemble of atoms can be described
by a simple Hamiltonian and has been extensively
studied. Nonetheless, the vector properties of light
in conjunctions with the multilevel structure of real
atoms and their collective response result in rich and
surprising physics. In our group, we investigate this
subject matter using nanophotonic components, such as
subwavelengthdiameter optical fibers and
whisperinggallerymode resonators, for interfacing
light and atoms. I will present three effects that we
observed in experiments with these systems and that go
beyond the standard description of lightmatter
coupling. First, transversally confined light can
locally carry transverse spin angular momentum, which
leads to propagation directiondependent emission and
absorption of light. Second, when imaging an
elliptically polarized emitter with a perfectly
focused, aberrationfree imaging system, its apparent
position differs from the actual position. Third, an
ensemble of atoms can change the photons statistics of
light transmitted through the ensemble. There,
depending on the number of coupled atoms, a
collectively enhanced nonlinearity leads to pronounced
photon bunching or antibunching.
Thu 20200625 17:00 CEST
Quantum Science Seminar #10: Ultrafast Science
Lund University
Lund — Sweden
Lund — Sweden
Atomic photoionization using attosecond pulses
Since the beginning of the millennium, physicists know
how to generate pulses of light of attosecond duration
[1], thus gaining access to this incredibly short time
scale. In this presentation, we will show how
attosecond pulses bring new light on ultrafast
electron dynamics in atomic photoionization. We use
attosecond pulse trains together with a weak infrared
probe to measure both amplitude and phase of
photoionization matrix elements. Our method, which
combines high temporal and spectral resolution [2],
allows us to gain new insights on photoionization
dynamics, including electron correlation and spin flip
induced by spinorbit interaction. In another
experiment, we characterize an electron wavepacket
near an autoionizing resonance in helium using a
Wigner representation [3], and retrieve the
corresponding timedependent density matrix.
References
Thu 20200618 17:00 CEST
Quantum Science Seminar #09: Molecules
JILA, NIST and University of Colorado
Boulder — Colorado — U.S.A.
Boulder — Colorado — U.S.A.
A Fermi gas of polar molecules from 3D to 2D
Quantum degenerate gases of polar molecules provide a
new platform for quantum science [1]. A Fermi gas of
KRb molecules is fully thermalized with atommolecule
interactions and characterized using thermometry based
on suppressed density fluctuations [2]. To demonstrate
the full potential of strong dipolar interactions in
the molecular gas, we apply external electric fields
to explore the exciting interplay between molecular
interaction dynamics and dissipation. By confining KRb
to two dimensional optical traps with a perpendicular
electric field [3], we demonstrate greatly enhanced
elastic collisions with strong suppression of
inelastic loss, with their ratio reaching 100. The
favorable 2D dipolar interactions have led to rapid
thermalization and evaporation of molecules.
References
Thu 20200611 17:00 CEST
Quantum Science Seminar #08: Quantum Transport
University of Geneva
Geneva — Switzerland
Geneva — Switzerland
Quantum transport, low dimensions and cold atomic systems
Measuring the transport properties of a system
connected to reservoirs is one of the most common and
most useful probe of the properties of a solid.
Besides its practical interest transport in quantum
systems poses fundamental and challenging theoretical
questions, since it is one of the simplest
realizations of an out of equilibrium phenomenon. I
will review these issues, in particular in the case of
one and quasione (e.g. ladders) systems. In such
systems we know that interactions lead to unusual
ground states [12] and remarkable properties such as
spincharge decoupling, which of course has strong
consequences for transport properties, in particular
decoupling charge and spin transport [3]. I will
connect these theoretical questions with experiments
done in the context of cold atomic systems that
provide novel ways to probe such physics [4].
References

Quantum Physics in One DimensionOxford University Press20039780198525004

Interactions in Quantum FluidsLecture Notes of the Les Houches Summer School913962011

Spin transport in a onedimensional quantum wirePhysical Review Research20230622020

Band and correlated insulators of cold fermions in a mesoscopic latticePhysical Review X80110532018
Thu 20200528 17:00 CEST
Quantum Science Seminar #07: Photonics
University of Utrecht
Utrecht — Netherlands
Utrecht — Netherlands
Quantum Fractals
The human fascination for fractals dates back to the
time of Christ, when structures known nowadays as a
Sierpinski gasket were used in decorative art in
churches. Nonetheless, it was only in the last century
that mathematicians faced the difficult task of
classifying these structures. In the 80’s and 90’s,
the foundational work of Mandelbrot triggered enormous
activity in the field. The focus was on understanding
how a particle diffuses in a fractal structure.
However, those were classical fractals. This
century, the task is to understand quantum
fractals. Last year, we experimentally realized a
Sierpinski gasket using a scanning tunneling
microscope to pattern adsorbates on top of Cu(111) and
showed that the wavefunction describing electrons in a
Sierpinski gasket fractal has the Hausdorff dimension
d = 1.58 [1,2,3]. However, STM
techniques can only describe equilibrium
properties.
Now, we went a step beyond and using stateoftheart photonics experiments, we unveiled the quantum dynamics in fractals. By injecting photons in waveguide arrays arranged in a fractal shape, we were able to follow their motion and understand their quantum dynamics with unprecedented detail. We built and investigated 3 types of fractal structures to reveal not only the influence of different Hausdorff dimension, but also of geometry [4]. Finally, I will tell you about the dynamics of systems governed by a fractional Langevin equation. It turns out that this kind of approach may describe the Gardner phase in glasses, which is a phase exhibiting a fractal structure in the free energy landscape. We find an anomalous diffusion and reveal the existence of a novel regime, characterizing a Time Glass [5].
Now, we went a step beyond and using stateoftheart photonics experiments, we unveiled the quantum dynamics in fractals. By injecting photons in waveguide arrays arranged in a fractal shape, we were able to follow their motion and understand their quantum dynamics with unprecedented detail. We built and investigated 3 types of fractal structures to reveal not only the influence of different Hausdorff dimension, but also of geometry [4]. Finally, I will tell you about the dynamics of systems governed by a fractional Langevin equation. It turns out that this kind of approach may describe the Gardner phase in glasses, which is a phase exhibiting a fractal structure in the free energy landscape. We find an anomalous diffusion and reveal the existence of a novel regime, characterizing a Time Glass [5].
References

Design and characterization of electrons in a fractal geometryNature Physics151272019

Quantum corral herds surface electrons into a fractal latticePhysics Today72142019

Scientists Trapped Electrons In a Quantum Fractal (And It's Wild!)Youtube2019

Shining light on quantum transport in fractal networks2020

Time Glass: a fractional calculus approach2020
Thu 20200521 17:00 CEST
Quantum Science Seminar #06: Molecules
Harvard University
Cambridge — Massachusetts — U.S.A.
Cambridge — Massachusetts — U.S.A.
Combining Chemistry and Physics in Ultracold Polar Molecules
Advances in quantum manipulation of molecules bring
unique opportunities, including the use of molecules
to search for new physics, harnessing molecular
resources for quantum engineering, and exploring
chemical reactions in the ultralow temperature
regime. In this talk, I focus on the latter topic
where we work toward a detailed microscopic picture of
molecules transforming from one species to another and
reveal several surprises along the way. By preparing
quantumstateselected KRb molecules at a temperature
of 500 nK, we observed reactions proceeding through a
longlived intermediate, which provides a handle to
steer with light the reaction pathway away from its
natural course. Despite the long lifetime that might
allow thermalization, our measurements indicate that
ergodicity does not hold for all degrees of freedom.
References

Ultracold chemistry: No longer a disappearing actPhysics Today73122020

Steering ultracold reactions through longlived transient intermediatesarXiv2002.051402020

Forming a single molecule by magnetoassociation in an optical tweezerarXiv2003.078502020

Dipolar exchange quantum logic gate with polar moleculesChemical Science968302018
Thu 20200514 17:00 CEST
Quantum Science Seminar #05: Quantum Optics
Aarhus University
Aarhus — Denmark
Aarhus — Denmark
Quantum interactions with radiation that moves
How does a quantum system interact with a travelling
pulse of quantum radiation, prepared, e.g., in a
number state or a coherent state of light? You may
think that this problem has been text book material
for decades along with detailed solutions for the case
of simple, few level systems. But, in fact, it has
not. While crucial for multiple effects in quantum
optics and for the entire concept of flying and
stationary qubits, quantum optics textbooks do not
provide a formal description applicable to this
foundational and elementary interaction process. After
the introduction of a new (and simple) theoretical
formalism that, accounts for the interaction of
travelling pulses of quantized radiation with a local
quantum system, I shall discuss applications of the
theory to quantum pulses of optical, microwave and
acoustic excitations and show examples of relevance to
recent experiments with qubits and nonlinear
resonators.
References

Jaynes–Cummings model — Schrödinger picture dynamicsWikipedia20200510

InputOutput Theory with Quantum PulsesPhysical Review Letters1231236042019

Quantum interactions with pulses of radiationarXiv2003.045732020

Scattering into onedimensional waveguides from a coherentlydriven quantumoptical systemQuantum2692018
Thu 20200507 17:00 CEST
Quantum Science Seminar #04: Polaritons
Center for Nanoscience and Nanotechnology
C2N — Université Paris Saclay — CNRS
Palaiseau — France
C2N — Université Paris Saclay — CNRS
Palaiseau — France
Quantum fluids of light in semiconductor lattices
When confining photons in semiconductor lattices, it
is possible to strongly modify their physical
properties and explore the physics of a variety of
Hamiltonians. Photons can behave as finite or even
infinite mass particles, photons can propagate along
topological edge states without back scattering,
photons can become superfluid and behave as massive
interacting particles. These are just a few examples
of exotic properties that we can imprint into quantum
fluids of light in semiconductor lattices. Such
manipulation of light presents not only potential for
applications in photonics, but great promise for
fundamental studies of driven dissipative systems.
After a detailed introduction to quantum fluids of
light, I will illustrate the variety of physical
systems we can emulate with this photonic platform by
presenting some recent experiments related to
quasicrystals, helical photons, and photonic
graphene. Perspectives in terms of quantum
correlations will be discussed.
Thu 20200430 17:00 CEST
Quantum Science Seminar #03: Quantum Optics
Institute of Photonic Sciences (ICFO)
Barcelona — Spain
Barcelona — Spain
The maximum refractive index of an atomic medium
It is interesting to observe that all optical
materials with a positive refractive index have a
value of index that is of order unity. Surprisingly,
though, a deep understanding of the mechanisms behind
this universal behavior seems to be lacking. Moreover,
this observation is difficult to reconcile with the
fact that a single, isolated atom is known to have a
giant optical response, with a resonant scattering
cross section that far exceeds its physical size.
Here, we theoretically investigate the evolution of the optical properties of an atomic ensemble as a function of increasing density, including the effects of multiple scattering and nearfield interactions. We find that the index does not grow indefinitely with density, but rather reaches a limiting value of n ~ 1.7. Using strongdisorder renormalization group theory, we show that this maximum value arises from the combination of random atomic positions and nearfield interactions, which results in a inhomogeneous broadening of atomic resonance frequencies. Thus, regardless of the physical atomic density, light at any given frequency only interacts with approximately one nearresonant atom per cubic wavelength, limiting the maximum index attainable. Finally, we discuss how this simple atomic physics limit might be extended to arrive at a theory for reallife solids.
Here, we theoretically investigate the evolution of the optical properties of an atomic ensemble as a function of increasing density, including the effects of multiple scattering and nearfield interactions. We find that the index does not grow indefinitely with density, but rather reaches a limiting value of n ~ 1.7. Using strongdisorder renormalization group theory, we show that this maximum value arises from the combination of random atomic positions and nearfield interactions, which results in a inhomogeneous broadening of atomic resonance frequencies. Thus, regardless of the physical atomic density, light at any given frequency only interacts with approximately one nearresonant atom per cubic wavelength, limiting the maximum index attainable. Finally, we discuss how this simple atomic physics limit might be extended to arrive at a theory for reallife solids.
Thu 20200423 17:00 CEST
Quantum Science Seminar #02: Quantum Computing
Weizmann Institute of Science
Rehovot — Israel
Rehovot — Israel
Trappedion quantum computing: a coherent control problem
Several systems have been investigated in the last
couple of decades as possible platforms for the
realization of a quantum computer. Among the different
systems examined, trappedion systems have thus far
demonstrated the highest fidelity quantum gates and
very long coherence times. However, scaling
trappedion quantum computers to large numbers of
qubits has proven to be a difficult problem. In this
talk I will review the quantum toolbox of trappedion
quantum computing and discuss the coherent control
techniques that were developed in recent years that
render trappedion quantum gates robust against errors
and allow for quantum computing on long chains of
ions.
Thu 20200416 17:00 CEST
Quantum Science Seminar #01: Quantum Simulation
MaxPlanckInstitute of Quantum Optics (MPQ)
Garching b. München — Germany
Garching b. München — Germany
Analog Quantum Simulation: from physics to chemistry
Manybody systems are
very hard to simulate due to the explosion of parameters with the
system size. Quantum computers can help in this task, although one may
need scalable systems, something that is out of reach in the short
run. An attractive alternative is provided by analog quantum
simulators which, even though they are not universal, they can still
be tuned to study interesting problems. Atoms in optical lattices seem
to be ideally suited for that task. Most of the proposals of such
simulators have focused so far on condensed matter or high energy
physics problems. In this talk I will show how one can extend the
range of problems to other scenarios, especially to quantum chemistry.